The present invention relates to thermographic nondestructive testing techniques for determining the thickness of an object. More particularly, the present invention relates to a high speed infrared transient thermography method and apparatus for accurately measuring the wall thickness of metal turbine rotor blades or the like.
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine cross-sectional thickness of cast metal and other solid objects. Conventionally, the object is probed with ultrasonic waves which penetrate the surface and are reflected internally at the opposite side or surface of the object. Based upon the time required to receive a reflected wave, the distance to the opposite (back) side can be determinedxe2x80x94giving the thickness of the object at that point. Unfortunately, conducting ultrasonic measurements of this sort to examine the cross-sectional thickness for most of an object would usually necessitate a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface, a stream of liquid couplant must be applied to the surface or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are most often not very practical or even feasible for numerous structural and material reasons. For example, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can literally take hours.
Moreover, when conducting ultrasonic measurements on certain metal objects, the internal crystal orientation and structure of the metal can cause undesirable noise and directional effects that contribute to inaccuracies in the acquired data. This inherent limitation of ultrasonic measurements proves to be a serious drawback when testing components constructed of crystalline or xe2x80x9cdirectionalxe2x80x9d metals such as often used in contemporary turbine airfoils.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Since heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. In contrast to most ultrasonic techniques, a transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one-hundred times faster than most conventional ultrasonic methods if testing objects of large surface area.
One known contemporary application of transient thermography, which provides the ability to determine the size and xe2x80x9crelativexe2x80x9d location (depth) of flaws within solid non-metal composites, is revealed in U.S. Pat. No. 5,711,603 to Ringermacher et al., entitled xe2x80x9cNondestructive Testing: Transient Depth Thermographyxe2x80x9d; and is incorporated herein by reference. Basically, this technique involves heating the surface of an object of interest and recording the temperature changes over time of very small regions or xe2x80x9cresolution elementsxe2x80x9d on the surface of the object. These surface temperature changes are related to characteristic dynamics of heat flow through the object, which is affected by the presence of flaws. Accordingly, the size and a value indicative of a xe2x80x9crelativexe2x80x9d depth of a flaw (i.e., relative to other flaws within the object) can be determined based upon a careful analysis of the temperature changes occurring at each resolution element over the surface of the object. Although not explicitly disclosed in the above referenced Ringermacher patent, the xe2x80x9cactualxe2x80x9d depth of a flaw (i.e., the depth of a flaw from the surface of the object) can not be determined unless a xe2x80x9cstandards blockxe2x80x9d, having voids at known depths, or an xe2x80x9cinfinitexe2x80x9d (thermally thick) reference region on the object is included as part of the thermographic data acquisition and analysis for comparison against the relative depth values.
To obtain accurate thermal measurements using transient thermography, the surface of an object must be heated to a particular temperature in a sufficiently short period of time so as to preclude any significant heating of the remainder of the object. Depending on the thickness and material characteristics of the object under test, a quartz lamp or a high intensity flash-lamp is conventionally used to generate a heat pulse of the proper magnitude and duration. However, the specific mechanism used to heat the object surface could be any means capable of quickly heating the surface to a temperature sufficient to permit thermographic monitoringxe2x80x94such as, for example, pulsed laser light). Once the surface of the object is heated, a graphic record of thermal changes over the surface is acquired and analyzed.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame which corresponds to a rectangular area, called a xe2x80x9cresolution elementxe2x80x9d, on the surface of the object being imaged. Since, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast. The stored video images are used to determine the contrast of each pixel in an image frame by subtracting the mean pixel intensity for a particular image frame, representing a known point in time, from the individual pixel intensity at that same point in time.
The contrast data for each pixel is then analyzed in the time domain (i.e., over many image frames) to identify the time of occurrence of an xe2x80x9cinflection pointxe2x80x9d of the contrast curve data, which is mathematically related to a relative depth of a flaw within the object. Basically, as applied to an exemplary xe2x80x9cplatexe2x80x9d like object of consistent material and thickness L, a heat flux pulse impinging on an object takes a certain xe2x80x9ccharacteristic timexe2x80x9d, Tc, to penetrate through the object to the opposite side (back wall) and return to the front surface being imaged. This characteristic time, Tc, is related to the thickness of the object, given the thermal diffusivity of the material by the following equation:
Tc=4L2/xcfx802xcex1xe2x80x83xe2x80x83Equ. (1)
where L is the thickness (cm) of the object and a is the thermal diffusivity (cm2/sec) of the material.
From empirical observations it is known that after a heat pulse impinges on a plate-like object, the surface temperature observed from the same side of the object (i.e., the front) rises in a fashion that is also dependent on the thickness and the thermal diffusivity of the material. Moreover, from a graph of the time vs. temperature (T-t) history of the surface, one can determine the characteristic time, Tc, in terms of a unique point on the T-t curve, called the xe2x80x9cinflection point.xe2x80x9d This inflection point, tinfl, is indicated by the point of maximum slope on the T-t curve (i.e., peak-slope time) and is related to the characteristic time, Tc, by the following equation:
tinfl=0.9055 Tcxe2x80x83xe2x80x83Equ. (2)
This relationship between the inflection point and the characteristic time, as expressed by Equ. (2) above, is precise to approximately 1% for one-dimensional (1-D), as well as two-dimensional (2-D), heat flow analysis. Once an inflection point, tinfl, is determined from the T-t response, a relative thickness, L, of the object can be determined from Equ. (1) using the known thermal diffusivity, xcex1, of the material and the actual value of Tc from Equ (2).
In this regard, a more detailed discussion of the heat-flow invariant relationship between the peak-slope time (inflection point) and the material xe2x80x9ccharacteristic timexe2x80x9d as defined above may be found in the Review Of Progress In Quantitative Nondestructive Evaluation, in an article by Ringermacher et al., entitled xe2x80x9cTowards A Flat-Bottom Hole Standard For Thermal Imagingxe2x80x9d, published May 1998 by Plenum Press, New York, which is incorporated herein by reference.
Unfortunately, although the above referenced Ringermacher et al. patented method for flaw detection may be effective on ceramics, plastics, composites and other non-metallic objects, it is not particularly feasible for use in determining the thickness of metal objects. One of the problems is that metals have significantly higher thermal conductivity and thermal diffusion characteristics than non-metals. This decreases the time period during which useful thermal data can be acquired and requires increasing the sensitivity of the IR recording equipment. The increased sensitivity of the equipment causes inaccuracies in the acquired data arising from IR noise from other ambient IR sources such as the flash-lamps. Moreover, since the apparatus and method of the above referenced Ringermacher et al. patent only produces xe2x80x9crelativexe2x80x9d measurements, it can not be used to obtain a value for the actual thickness of a metal object at a desired point. Consequently, an improved method of conducting and processing IR transient thermography for determining the actual thickness of metal objects is needed.
The present invention relates to a nondestructive testing method and apparatus for determining and displaying the actual thickness of an object through the use of high speed infrared (IR) transient thermography. A unique aspect of the present invention is that the object of interest can be a metal material, even though metals have high rates of thermal conduction.
In accordance with the present invention, an improved high speed IR transient thermography arrangementxe2x80x94based upon known heat flow dynamics within metal objectsxe2x80x94is utilized to accurately measure the thickness of a metal object and provide a visual color-coded display indicative of its cross-sectional thickness over a desired area of the object. Moreover, the improved transient thermographic techniques of the present invention can be used to measure the thickness of metal objects with greater accuracy than conventional ultrasonic methods.
In this regard, the present invention makes particular use of the inflection point in the analysis of metal objects because the inflection point occurs relatively early in the T-t response and is essentially independent of lateral heat loss mechanisms. Such considerations are important when working with metal objects since, due to the high thermal conductivity of metals, the thermal response is fast and, thus, the available observation time will be short. Moreover, in accordance with the present invention, when a T-t curve is obtained from xe2x80x9cfront-sidexe2x80x9d IR camera observations, a more accurate determination of the inflection point is obtained if an image xe2x80x9ccontrastxe2x80x9d curve is created by subtracting the observed T-t curve from a hypothetical or measured T-t curve reflecting a deep interior xe2x80x9creference regionxe2x80x9d within the heated object (this reference region approximating an xe2x80x9cinfinitely thickxe2x80x9d object) or in an adjacent thermally thick xe2x80x9creferencexe2x80x9d block.
With the present invention, the inflection point, tinfl, is determined from thermal data obtained over a predetermined time period from successive IR camera image frames. This time period is preferably at least somewhat longer than an anticipated characteristic time, as obtained from Equ. (1), based on an estimation of the thickness of the object being evaluated.
Unfortunately, as recognized by the inventors of the present invention, the known prior art IR transient thermography apparatus has at least one drawback: the flash-lamps required for generating the heating pulse produce long-wave IR xe2x80x9cafterglowxe2x80x9d emissions that ultimately reach the object and the camera as background radiation. This results in a reduced contrast within the recorded IR images and ultimately affects the accuracy of the thermal measurements. Accordingly, the present invention provides an improved high speed IR transient thermography apparatus that substantially eliminates problems associated with xe2x80x9cafterglowxe2x80x9d IR emissions caused by conventional flash-lamp heating arrangements. In addition, the present invention provides an improved IR transient thermographic method and apparatus for effective quantitative evaluation of the front wall thickness of metal objects such as turbine rotor blades or the like. Furthermore, the present invention provides an improved method and apparatus for determining the thickness of a metal object, wherein the means for acquiring data for determining the thickness is substantially unaffected by the particular internal crystalline structure of the object, such as are common to metal castings and other xe2x80x9cdirectionalxe2x80x9d metal objects.
In accordance with a preferred embodiment of the present invention, the apparatus includes an imaging system comprising one or more high power flash lamps fitted with special optical filters, an IR sensitive focal-plane array camera for data acquisition and a display monitor. A computer system controls the imaging system, records and analyzes surface temperature data acquired via the IR camera and provides a color-keyed image on the display monitor that accurately corresponds to thickness of the object.
The acquisition of surface temperature data is initiated by firing the flash-lamps to illuminate the surface of the object. The special optical filters are spectrally tuned to absorb and/or reflect all 3-5 micron IR radiation back into the flash-lamp(s). This prevents undesirable long-wave IR xe2x80x9cafterglowxe2x80x9d emissionsxe2x80x94typically generated by overheated metallic elements in the flash-lamps after the lamps are extinguishedxe2x80x94from reaching the object or the camera. The use of such filters enables a more precise thermal evaluation that can produce dimensional measurements within an accuracy range of 1%-3% of actual thickness.
A predetermined number of image frames are then recorded over a period of time after the flash lamps are fired and the recorded images used to develop a temperature-time (T-t) history for every elemental region or xe2x80x9cresolution elementxe2x80x9d over the region of interest on the object surface. Each recorded image frame is comprised of a predetermined (nxc3x97m) array of image pixels whose intensity correlate to the surface temperature of the object at the time the frame data was acquiredxe2x80x94each pixel having an (x,y) location designation within the image frame that corresponds to a particular resolution element.
A heat flow analysis of the T-t history is then conducted for each pixel in the acquired image frames to determine the thickness of the object at each resolution element location (either a one-dimensional or a multi-dimensional heat flow analysis approach may be used). Conventionally, analysis of transient heat flow through solid portions of an object requires determining the characteristic time, Tc, required for a xe2x80x9cpulsexe2x80x9d of thermal energy to penetrate the object at a first surface, reflect off an opposite surface and return to the first surface. Since the characteristic time is related to the distance between the two surfaces, it can be used to determine the thickness of the object between the two surfaces at a desired point. Fortunately, a value for characteristic time Tc may be determined from the thermographic T-t history of a pixel since it is related in time to the occurrence of an xe2x80x9cinflection pointxe2x80x9d in the recorded contrast history of the pixel according to Equ. (2) above.
An improved heat flow analysis method is also provided in accordance with the present invention that, among other things, facilitates a more accurate determination of the contrast history xe2x80x9cinflection point.xe2x80x9d As described in greater detail below, a contrast curve is first determined for each (x,y) pixel location corresponding to each resolution element of the object surface. This contrast curve is formulated based on an average pixel intensity obtained using a xe2x80x9cthermally thickxe2x80x9d portion (in a preferred embodiment the thermally thick portion is at least five (5) times as thick as the total thickness of the object being measured) of the object or of a different xe2x80x9creferencexe2x80x9d block having thermal conductivity similar to the object being measured. In a preferred embodiment, a separate thermally thick block is positioned adjacent the object being measured and imaged together with it. In another embodiment, a xe2x80x9cstandardsxe2x80x9d block having a thermally thick portion and comprising different steps of known thickness is thermographically imaged together with the object of interest.
Next, Gaussian temporal smoothing of the pixel contrast curve data is employed to improve the signal-to-noise ratio of the measurements. The mathematical derivative of the contrast curve is then calculated using three-point data sampling having a first and third sample point separation that is proportionally related to the value of the image frame number at the second sample point. Next, local peaks in the derivative of the contrast curve data are determined and a weighting function is used to adjust the significance of each peak to identify a best xe2x80x9cinflection pointxe2x80x9d in the T-t contrast curve data for use in determining object thickness. Correction of image pixel data is also employed to offset the effects of varying IR emissivity due to surface curvatures of the imaged object.